Biological Half-life

The radioactive half-life for a given radioisotope is physically determined and unaffected by the physical or chemical conditions around it. However, if that radioisotope is in a living organism it may be excreted so that it no longer is a source of radiation exposure to the organism. For a number of radioisotopes of particular medical interest, the rate of excretion has been cast in the form of an effective biological half-life. The rate of decrease of radiation exposure is then affected by both the physical and biological half-life, giving an effective half-life for the isotope in the body. Though the biological half-life cannot be expected to be as precise as the physical half-life, it is useful compute an effective half-life from

1/TEffective = 1/TPhysical +1/TBiological

Examples of the half-lives show that biological clearing is sometimes dominant and sometimes physical decay is the dominant influence.

Isotope
Half-lives in days
TPhysical
TBiological
TEffective
3H
4.5 x 103
12
12
32P
14.3
1155
14.1
90Sr
1.1 x 104
1.8 x 104
6.8 x 103
99mTc
0.25
1
0.20

Tritium, 3H, has a fairly long physical halflife but clears from the body quickly, lessening the exposure. Phosphorous, 32P, is used for some kinds of bone scans. The phosphorous tends to be held in the bones, leading to a long biological half-life, but its physical half-life is short enough to minimize exposure. Strontium, 90Sr, is very bad news in the environment. It mimics calcium and therefore gets trapped in bone. This gives it a long biological half-life to go with its long physical half-life, making it doubly dangerous. Technetium, 99mTc, is one of the favorites for diagnostic scans because of short physical and biological half-lives. It clears from the body very quickly after the imaging procedures.

Table of Physical, Biological, and Effective Half-lives
Index

Nuclear applications to health
 
HyperPhysics***** Nuclear R Nave
Go Back





Physical, Biological, and Effective Half-lives for Selected Isotopes

The radioactive half-life and the biological half-life for a given radioisotope act together to reduce the radiation exposure from a given radioisotope. The following data for some radioisotopes of medical or general interest was taken from Tuszynski and Dixon's "Biomedical Applications of Introductory Physics".

Isotope
Half-lives in days
TPhysical
TBiological
TEffective
3H
4.5 x 103
12
12
14C
2.1 x 106
40
40
22Na
850
11
11
32P
14.3
1155
14.1
35S
87.4
90
44.3
36Cl
1.1 x 108
29
29
45Ca
165
1.8 x 104
164
59Fe
45
600
42
60Co
1.93 x 103
10
10
65Zn
244
933
193
86Rb
18.8
45
13
90Sr
1.1 x 104
1.8 x 104
6.8 x 103
99mTc
0.25
1
0.20
123I
0.54
138
0.54
131I
8
138
7.6
137Cs
1.1 x 104
70
70
140Ba
12.8
65
10.7
198Au
2.7
280
2.7
210Po
138
60
42
226Ra
5.8 x 105
1.6 x 104
1.5 x 104
235U
2.6 x 1011
15
15
239Pu
8.8 x 106
7.3 x 104
7.2 x 104
Index

Nuclear applications to health
 
HyperPhysics***** Nuclear R Nave
Go Back